Polypropylene is a well known commercial polymer, used for a variety of products such as packaging films and extruded and molded articles. It is produced by polymerization of propylene over transition metal coordination catalysts, specifically titanium halide containing catalysts. Commercial polypropylene, however, is deficient in resistance to impact at low temperatures, i.e., 0.degree. C. and below (e.g. -30.degree. C.). Polypropylene is disadvantaged in this low temperature toughness because of its comparatively high glass transition temperature. The several conventional methods of dealing with this deficiency for uses requiring low temperature exposure involve: (1) blending of homopolymer polypropylene with other polymers of lower Tg; (2) sequential ("block") copolymerization where the homopolymerization step is followed by a stage of copolymerization (usually ethylene-propylene) to produce what amounts to an in-situ reactor blend; and (3) a variety of hybrid combinations of blending and copolymerization. In all cases the blended-in or copolymerized component is rubbery or highly elastomeric in character. The criterion for low temperature toughness generally accepted for polypropylene by industry is a high value in the Gardner falling weight impact test at -30.degree. C.; 25 in-lb is a minimum for a medium impact rating and 150 in-lb for a high impact rating.
Blending and sequential copolymerization can be called external plasticization, but do not represent plasticization in the strict sense since they do not normally produce more than a minor shift in the Tg of the host homopolymer. This is because, with scarce exception, the copolymer "block" segments and blended-in polymeric components are incompatable with the host polymer so that flexibilization and low temperature properties derive from the mechanics of matrix-dispersed (soft) phase morphology rather than through specific interactions with the host polymer at the intermolecular level. "True" external plasticization, on the other hand, usually involves relatively strong interactions between polymer chain segments and (normally) low molecular weight molecules or oligomers which share some compatabilizing functionality. Examples of high external plasticization activity in polyolefins are scarce since intermolecular polymer-plasticizer interactions are limited to van der Waals' forces. It is to be expected, of course, that where high activity is found for a polyolefin, high impact strength can be imparted at a particular temperature if the Tg can be shifted to below the temperature of the test.
One approach to plasticizing polypropylene is disclosed in U.S. Pat. No. 3,201,364. In that patent polypropylene is blended with materials having a solubility parameter between 7.0 and 9.5 and a boiling point of over about 200.degree. C. In U.S. Pat. No. 3,201,364, none of the materials cited, with the possible exception of "Nujol" mineral oil, are effective plasticizers of polypropylene in the sense of producing the degree of low temperature shift of Tg necessary to effect the levels of Gardner impact strength at -30.degree. C. required to qualify even as medium impact polypropylene. In fact, most of the materials cited produce little if any shift in the Tg of highly isotactic polypropylene. Furthermore, even "Nujol", if it were a light enough mineral oil to effect the necessary Tg shift, would not produce the requisite Gardner impact strength by itself. It should be noted that the Gardner test is a very much more demanding measure of low temperature toughness, and relates much more reliably to real field performance than the flexural tests employed in U.S. Pat. No. 3,201,364. Note also the no impact tests of any kind are cited for support in the examples of U.S. Pat. No. 3,201,364.
We have discovered a new polymeric composition not having the disadvantages in the prior "plasticized" blends.